1. Field of the Invention
The present invention relates to a method for stencil plate making of a stencil sheet for stencil printing. More particularly, it relates to a method for stencil plate making of a stencil sheet for stencil printing using a micro porous plastic sheet as a stencil sheet for stencil printing and wherein the micro pores of the plastic sheet are closed by a thermal print head (referred to as TPH hereinafter).
2. Description of the Prior Art
Conventionally, as a sheet (stencil sheet) for stencil printing, a heat-sensitive sheet perforated by infrared radiation or TPH has been known. A sheet made of a thermoplastic film and a porous tissue paper stuck together by adhesive is used in general.
Further, rotary-type and simple press-type are known as a stencil plate printing machine using a heat-sensitive stencil sheet.
These printing machines perform printing by pushing the ink from the tissue paper side of the stencil sheet through pores perforated in the film corresponding to a line area of a print image thereby transferring ink onto print paper.
In the conventional stencil printing system, an improvement in ink drying has been sought because it takes a long time for ink to permeate print paper.
Namely, there are problems such that ink hardly permeates the surface of print paper, making fingers stained upon touching a printed sheet right after printing. Another problem is that when printing with a second color and subsequent colors in multicolor printing or printing on the back of the printed surface of the paper continuously, the ink is transferred to a rubber roller of the printing machine, thereby making the printed sheet stained owing to the ink transferred. The problem causes another disadvantage that a long time (10 to 20 minutes, for example) is needed to move to a further step.
To solve the above problem for obtaining instant dryness, it is effective to enhance ink permeability for print paper using low viscosity ink, to facilitate drying.
However, even if low viscosity ink is used in case that the amount of ink transferred is excessive, its drying worsens Therefore, in the conventional stencil plate printing system, when low viscosity ink is used, in order to suppress the amount of ink transferred, perforation diameter is required to be at least smaller than 20 xcexcm.
However, when the perforation diameter is decreased as described above, heat element density (resolution) of TPH must be increased to make that a line are not fainted by increasing dot density to be perforated.
This would raise the cost of TPH as well as require level improvement in peripheral techniques such as securing of durability of TPH, yield improvement and increasing film sensitivity of a heatsensitive stencil sheet.
To solve the above problems, the present inventors have proposed a method for stencil plate making which comprising: preparing a stencil plate having a large number of continuous pores in the order of sub-micron (referred to as a micro porous sheet or a sheet hereinafter); and the pores corresponding to a non-line area are closed to obtain an ink impermeable area.
However, there is a problem that when making a stencil plate of a micro porous sheet in the aforementioned method, some pores without being closed (referred to as pinhole hereinafter) which are supposed to be closed by shrinking or melting a sheet, are produced in reality and that ink passes through the pinholes and transferred to printing paper.
A method for coating the surface of the sheet with a releasing agent is proposed to solve the problem. However, there is still a problem that some pinholes are produced even though the aforementioned method is used.
There is another problem that when the sheet is stencil plate made according to the above method, the sheet is shrunk with heat of TPH during stencil plate making, resulting in that the original dimension reproducibility is poor. Further, when the sheet is fed into the printing machine or attached to a printing drum, the sheet is wrinkled.
Namely, an object of the present invention is to provide a method for stencil plate making in which micro pores of a stencil sheet are closed by applying heat with TPH required for shrinking and melting the surface or up to the inside of the micro porous sheet, and wherein, no pinholes are produced; and excellent dimensional reproducibility of the sheet is obtainable.
To achieve the above object, the applicants have discovered the following conditions: driving conditions for TPH such as heating temperature, current-carrying time period and current-carrying cycle; a pressure condition (referred to as pressure condition for stencil plate making hereinafter) under which a sheet is pressed between TPH and its corresponding platen roller: and a thermal shrinkage condition of the sheet. These conditions allow producing no pinholes or very few pinholes on the sheet.
Further, the applicants have also discovered that when making a stencil plate, the rate of the thermal shrinkage of the micro porous sheet varies depending on TPH driving conditions, the stencil plate making pressure condition and the thermal shrinkage condition.
In other words, the method for stencil plate making of the present invention is characterized in that a stencil sheet is stencil plate made for a micro porous sheet by closing micro pores under the conditions which satisfies the following formulae at the same time.
 less than Driving Condition for TPH greater than 
xe2x88x9230xe2x89xa6Tpxe2x88x92Tmxe2x89xa6300 (xc2x0 C.)xe2x80x83xe2x80x83(1) 
(wherein Tp represents a heating peak temperature of TPH, and Tm represents a melting temperature (melting point) of the sheet)
10xe2x89xa6Toxc3x97100/Tsxe2x89xa680 (%)xe2x80x83xe2x80x83(2) 
(wherein To represents a current-carrying time period, Ts represents a current-carrying cycle, Toxc3x97100/Ts represents the ratio of To to Ts)
 less than Pressure Condition for Stencil Plate Making greater than 
0.1xe2x89xa6Pxe2x89xa61.0 (MPa)xe2x80x83xe2x80x83(3) 
(wherein P represents a pressure for stencil plate making)
 less than Thermal Shrinkage Condition greater than 
1xe2x89xa6STm-30xe2x89xa620 (%)xe2x80x83xe2x80x83(4) 
(wherein STm-30 represents thermal shrinkage ratio at a temperature 30xc2x0 C. lower than the melting point of the sheet Tm in TMA (thermal mechanical analysis)
Further, the applicants have discovered that a more preferable result can be obtained by controlling the distribution of heating temperature of TPH.
In other words, the present invention is characterized in that the ratio of the size and pitch of the heat element in the direction of main scanning and sub scanning satisfies the following formula (5) and (6). Provided that the direction of main scanning is the direction that heat element of TPH stand in line, and the direction of sub scanning is across the main scanning direction, that is, the direction stencil sheet is fed.
The present invention is characterized in that the ratio of the size of the heat element and the pitch of this heat element in the direction of the main scanning of the thermal print head is:
preferably
42xe2x89xa6MRS/MRPxe2x89xa688 (%)xe2x80x83xe2x80x83(5) 
and more preferably satisfied with
54xe2x89xa6MRS/MRPxe2x89xa688 (%) 
and the ratio of the size of the heat element and the pitch of this heat element in the direction of the sub-scanning of the thermal print head is:
preferably
42xe2x89xa6SRS/SRPxe2x89xa6519 (%)xe2x80x83xe2x80x83(6) 
and more preferably
54xe2x89xa6SRS/SRPxe2x89xa6330 (%) 
and furthermore preferably
65xe2x89xa6SRS/SRPxe2x89xa684 (%) 
(wherein MRS is the size of the heat element In the direction of main scanning and MRP is the pitch (length) of the heat element in the direction of main scanning of the thermal print head, SRS is the size of the heat element in the direction of sub-scanning and SRP is the pitch (length) of the heat element in the direction of sub-scanning of the thermal print head.)
In the case of a thick film type thermal print head, xe2x80x9cthe size of the heat element in the direction of main scanningxe2x80x9d is xe2x80x9cthe length between electrodes adjacent to each otherxe2x80x9d, and further, xe2x80x9cthe pitch (length) of the heat elementxe2x80x9d is xe2x80x9cthe pitch (length) of the electrodesxe2x80x9d.
It should be noted that the type of the thermal print head may be a line type of a thermal print head or a serial type of a thermal print head in the present invention. Moreover, the resistor of the thermal print head may be a thin film type thermal print head formed mainly by sputtering or a thick film type thermal print head formed by the method for thick film printing.
A mechanism for closing micro pores by heating in the present invention will be described hereinafter.
A micro porous plastic sheet thermally shrinks in its dimension from a lower temperature lower than the melting point of the sheet by heat generation of the heat element of TPH to release a residual stress caused by extension received during the time of making a film. At this event, although the micro pores are closed by the thermal shrinking, they are not completely closed when the thermal shrinking is not enough. As the temperature rises to reach the melting point of the sheet, the surface or up to the inside of the plastic sheet melts and then the multiple micro pores are completely closed, resulting in yielding complete blockage areas (non-line area). However, even if the temperature does not reach the melting point, and if the heat shrinking of the sheet is enough, at least micro pores on the face of the stencil sheet in contact with TPH are completely closed.
Making a stencil of the present invention is performed by nipping the sheet during applying pressure with TPH and a platen roller associated with TPH and driving the sheet. In other words, the sheet is maintained in a state of tension in which the thermal shrinkage is suppressed all the time by applying pressure. In this state, the sheet receives a shearing stress toward the sheet feeding direction of the plane by the pressure applied and the feeding the sheet. While the sheet before melting which begins shrinking or the sheet which has melted after the temperature reached its melting point, maintains its dimension accuracy to some extent because the sheet is nipped by using TPH and a plate roll. Moreover, the micro pores are stroked and closed by the shearing stress. Namely, the degree of blockage of the micro pores depends on the shearing stress, that is, the pressure condition for stencil plate making.
More particularly, the tensional state to suppress the thermal shrinkage of the sheet varies depending on the pressure condition for stencil plate making, resulting in that the thermal shrinkage of the sheet by making a stencil plate depends on the pressure condition for stencil plate making.
To be more precise, the degree of blockage of micro pores and the degree of thermal shrinkage of a sheet by making a stencil plate depend on the driving conditions for TPH such as heating temperature, current-carrying time period and current-carrying cycle of TPH, a pressure condition for stencil plate making, and a condition for thermal shrinkage.
Further, the degree of blockage of micro pores and the degree of thermal shrinkage of the sheet also depend on the distribution of heating temperature of the thermal print head (or the distribution of heating temperature on a micro porous sheet).
One of the factors for controlling the distribution of heating temperature of the thermal print head concerning the direction of main scanning of the thermal print head is xe2x80x9cthe ratio of the size of the heat element in the direction of main scanning to the pitch in the direction of main scanning of a heat elementxe2x80x9d. The distribution of heating temperature on the micro porous sheet depends on its ratio.
Further, concerning the direction of sub-scanning of the thermal print head, resolution (pitch) of element can be arbitrarily set by adjusting a feeding speed for a micro porous sheet (a stencil sheet) and the driving condition for the TPH such as current-carrying cycle. The distribution of heating temperature on a micro porous sheet varies depending on xe2x80x9cthe ratio of the size of the heat element in the direction of sub-scanning for the pitch (length) to the heat element in the direction of sub-scanningxe2x80x9d.
According to the method for stencil plate making of a stencil sheet for stencil printing of the present invention, excellent blockage of pores is achieved, and no pinholes or very few pinholes are produced. Moreover, the method can provide a stencil sheet with suppressed thermal shrinkage when making a stencil for stencil printing.
In addition to that, when stencil printing is performed by applying ink with low viscosity to a stencil plate according to the method for stencil plate making of the present invention, a printed matter with excellent image quality and instant drying is can be obtained.
The present disclosure relates to subject matter contained in Japanese Patent Application No. 2000-304072, filed on Oct. 3, 2000 and Japanese Patent Application No. 2000-333737, filed on Oct. 31, 2000, the disclosure of which is expressly incorporated herein by reference in its entirety.